Hybrid stent and method of making

ABSTRACT

A stent is formed by encasing or encapsulating metallic rings in an inner polymeric layer and an outer polymeric layer. At least one polymer link connects adjacent metallic rings. The stent is drug loaded with one or more therapeutic agent or drug, for example, to reduce the likelihood of the development of restenosis in the coronary arteries. The inner and outer polymeric materials can be of the same polymer or different polymer to achieve different results, such as enhancing flexibility and providing a stent that is visible under MRI, computer tomography and x-ray fluoroscopy.

This application is a continuation of U.S. application Ser. No.10/113,358, filed Apr. 1, 2002, now U.S. Pat. No. 7,691,461, whoseentire contents are incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

This invention relates to endoprosthesis devices, generally calledstents, and more particularly, to achieving desired visibility of suchdevices under magnetic resonance imaging (MRI), computer tomography, andfluoroscopy.

Stents are generally thin walled tubular-shaped devices composed ofcomplex patterns of interconnecting struts which function to hold open asegment of a blood vessel or other body lumen such as a coronary artery.They also are suitable for supporting a dissected arterial lining orintimal flap that can occlude a vessel lumen. At present, there arenumerous commercial stents being marketed throughout the world. Thesedevices are typically implanted by use of a catheter which is insertedat an easily accessible location and then advanced through thevasculature to the deployment site. The stent is initially maintained ina radially compressed or collapsed state to enable it to be maneuveredthrough the lumen. Once in position, the stent is deployed. In the caseof balloon expandable stents, deployment is achieved by inflation of aballoon about which the stent is carried on a stent-delivery catheter.

The stent must be able to simultaneously satisfy a number of mechanicalrequirements. First, the stent must be capable of withstanding thestructural loads, namely radial compressive forces, imposed on the stentas it supports the walls of a vessel lumen. In addition to havingadequate radial strength or more accurately, hoop strength, the stentshould be longitudinally flexible to allow it to be maneuvered through atortuous vascular path and to enable it to conform to a deployment sitethat may not be linear or may be subject to flexure. The material fromwhich the stent is constructed must allow the stent to undergo expansionwhich typically requires substantial deformation of localized portionsof the stent structure. Once expanded, the stent must maintain its sizeand shape throughout its service life despite the various forces thatmay come to bear thereon, including the cyclic loading induced by thebeating heart. Finally, the stent must be biocompatible so as not totrigger any adverse vascular responses.

In addition to meeting the mechanical requirements described above,there is a requirement that a stent be radiopaque or fluoroscopicallyvisible under x-rays. Accurate stent placement requires real timevisualization to allow the cardiologist or interventional radiologist totrack the delivery catheter through the patient=s vasculature andprecisely place the stent at the site of a lesion. This is typicallyaccomplished by fluoroscopy or similar x-ray visualization procedures.For a stent to be fluoroscopically visible it must be more absorptive ofx-rays than the surrounding tissue. This is typically accomplished bythe use of radiopaque materials in the construction of a stent, whichallows for its direct visualization. The most common materials used tofabricate stents are stainless steel and nickel-titanium alloys, both ofwhich are radiopaque. This factor, in combination with the radial wallthickness of about 0.002 to 0.009 inch of most stent patterns, rendersstents produced from these materials sufficiently radiopaque to beoptimally visualized with x-ray based fluoroscopy procedures. Althoughboth materials are generally regarded as being bio-compatible, somerecent concerns have arisen regarding the long term biocompatibility ofstainless steel. Over time, nickel, a constituent element of moststainless steels, tends to leach from a stainless steel stent. Inaddition, the chromium oxide layer present on the surface of stainlesssteel stents to prevent corrosion may have a tendency to degrade duringlong term use within the body.

Alternative, non-toxic, high density metals, such as cobalt-chromium,tantalum, iridium, platinum, gold, and the like, have been used in priorart stents. However, these alloys can sometimes either be excessivelyradiopaque or may lack sufficient strength for recoil, radial strengthrequirements, and long-term use in a dynamic vascular setting. Stentsconstructed of highly radiopaque materials appear overly bright whenviewed under a fluoroscope. This tends to overwhelm the image of thetissue surrounding the stent and obscures visualization of the stentlumen. Due to the lack of an appropriately radiopaque material, simplyconstructing a stent wholly out of a single material has heretofore notresulted in a stent with the optimal combination of mechanicalproperties and radiopacity. Thus, the art has moved in the direction ofcombining different materials to produce a mechanically sound,biocompatible and fluoroscopically visible stent. A number of suchapproaches have been developed. Typically such methods have focused onincreasing the radiopacity or fluoroscopic visibility of stainless steeland nickel-titanium alloy stents.

One means frequently described for increasing fluoroscopic visibility isthe physical attachment of radiopaque markers to the stent. Conventionalradiopaque markers, however, have a number of limitations. Uponattachment to a stent, such markers may project from the surface of thestent, thereby comprising a departure from the ideal profile of thestent. Depending on their specific location, the marker may eitherproject inwardly to disrupt blood flow or outwardly to traumatize thewalls of the blood vessel. Additionally, galvanic corrosion may resultfrom the contact of two disparate metals, i.e., the metal used in theconstruction of the stent and the radiopaque metal of the marker. Suchcorrosion could eventually cause the marker to separate from the stentwhich may be problematic should the marker be swept downstream within avessel. Discrete stent markers cannot show the entire outline of thestent which is a preferred method to determine the optimal expansion ofa stent over its entire length.

The radiopacity of stents has also been increased by plating or coatingselected portions thereof with radiopaque material. However, a number ofdisadvantages are associated with this approach as well. When the stentis expanded certain portions undergo substantial deformation, creating arisk that cracks may form in the plating or coating causing portions ofthe plating to separate from the underlying substrate. This has thepotential for creating jagged edges that may inflict physical trauma onthe lumen wall tissue or cause turbulence in the blood flowing past thestent, thereby inducing thrombogenesis. Moreover, once the underlyingstructural material becomes exposed to an electrolytic solution such asblood, interfaces between the two disparate metals become subject togalvanic corrosion. Over time, galvanic corrosion may also lead toseparation of the plated material from the underlying substrate.

X-ray based fluoroscopy is the current preferred modality for imagingstents during an intervention and for diagnostic assessment. However, inaddition to the potential disadvantages stated above, other drawbacksmay exist. Exposure to ionizing radiation and nephrotoxic iodinatedcontrast agents are intrinsic to the technique, as well as the need towear leaded personal protective equipment. Alternatively, magneticresonance imaging (MRI), produced by complex interactions of magneticand radio-frequency fields, does not suffer from these drawbacks and isactively being pursued to image stents in a diagnostic mode and, in thefuture, to guide stent based interventions. MRI has gained an increasingrole in the diagnosis and assessment of human pathology. In patientsundergoing MRI, there are numerous devices which are poorly seen, ifthey are visible at all, on the MR image artifact. The location andcourse of these implanted devices is usually of great clinicalimportance to assure their proper function and avoid complications thatmalposition can cause.

Due to their small size, current metal stents are sometimes difficult tosee in fluoroscopy as they attenuate the x-ray beam very little. This isparticularly true in very large, obese patients being imaged in lowerend grade imaging systems. In MRI, the problem is that ferromagnetic andmetallic based stents are difficult to see as they can create a largeimaging artifact (a region of signal void or diminishment, which canextend beyond the stent boundaries). A plastic medical device, namely apolymeric stent, is particularly better for MRI as it isnon-ferromagnetic and non-metallic. Indeed, a polymeric stent producessubstantially no artifact at all. The signal used in most conventionallyavailable MRI comes from the nuclear magnetic resonance of hydrogennuclei. Polymers contain hydrogen atoms but these nuclei resonate at afrequency which is shifted from the water hydrogen signal from which theimage is mainly derived. Moreover, the emitted RF signal from, polymersis quite broad. Under MRI, polymers appear as a region of signal voidthat is the same size as the device and therefore, more clinicallyaccurate. Unfortunately, this creates a situation analogous tofluoroscopy with a stent that is difficult to visualize. A solution toimaging a polymeric stent under MRI is to add a substance to the polymerto change its magnetic susceptibility. These materials are well known tothose skilled in the art and consist of paramagnetic or ferromagneticcompounds, particles and fillers. By the choice of agent, and itsconcentration in the polymer, the size of the susceptibility artifactcan be tuned.

Fluoroscopy generates a two-dimensional projection image of what arethree-dimensional structures. This requires multiple views to appraisecomplex vasculature. Another imaging modality, which has the potentialto supplant fluoroscopy and become important in the diagnostic imagingof stents, is magnetic resonance imaging (MRI). One advantage of MRI isthat it is a tomographic imaging technique that generates a 3-D data setof the imaged tissue. Consequently, the data set can be manipulated toshow different imaging planes and slice thicknesses. This permits highquality transverse, coronal and sagittal images to be obtained directly.MRI has greater soft tissue contrast and tissue discrimination thancomputed tomography (CT) or other x-ray based imaging modalities, suchas angiography. As another advantage, MRI also does not use ionizingradiation and does not require catheterization to image the vasculature.

The technique of MRI encompasses the detection of certain atomic nuclei(those possessing magnetic dipole moments) utilizing magnetic fields andradio-frequency (RF) radiation. It is similar in some respects to x-raycomputed tomography in providing a cross-sectional display of the bodyorgan anatomy, only with excellent resolution of soft tissue detail. Inits current use, the images constitute a distribution map of protons,and their properties, in organs and tissues. However, unlike x-raycomputer tomography, MRI does not use ionizing radiation. Thefundamental lack of any known hazard associated with the level of themagnetic and radio-frequency fields that are employed renders itpossible to make repeated scans. Additionally, any scan plane canreadily be selected, including transverse, coronal, and sagittalsections. MRI is, therefore, a safe non-invasive technique for medicalimaging.

The hydrogen atom, having a nucleus consisting of a single unpairedproton, has one of the strongest magnetic dipole moments of nuclei foundin biological tissues. Since hydrogen occurs in both water and lipids,it is abundant in the human body. Therefore, MRI is most commonly usedto produce images based upon the distribution density of protons and/orthe relaxation times of protons in organs and tissues. The majority ofthe signal in MRI comes from water. Tissues vary in their water content,but for angiography, blood is the relevant tissue. Blood isapproximately 93% water. This translates into a proton concentration of103 moles/liter. However, MRI can image tissues with a lower watercontent. For example, grey matter and bone are 71% and 12% waterrespectively. It must be noted that MRI can image proton concentrationsmuch lower than those of blood or grey matter. Image resolution isdetermined by the signal to noise (S/N) ratio. Faster acquisition ofdata or longer acquisition times both increase the signal to noiseratio.

MRI is presently used for diagnostic applications, but interventionalMRI is an active area of research. For devices to be seen under MRI,they must be MRI A compatible. @ In the context of a diagnostic orinterventional procedure, this refers to the ability to accurately imagea stent. MRI imaging schemes for devices are divided into twocategories, active and passive. Active imaging requires some sort ofelectrical circuit on, or electrical connection to, the device. Thispresently is not an easily implemented solution for small, free-standingdevices such as stents. The passive imaging scheme that applies readilyto metal stents is based on the stent material=s magnetic susceptibilityand electrical conductivity.

Because stents are constructed of electrically conductive materials,they suffer from a Faraday Cage effect when used with MRI=s.Generically, a Faraday Cage is a box, cage, or array of electricallyconductive material intended to shield its contents from electromagneticradiation. The effectiveness of a Faraday Cage depends on the wavelength of the radiation, the size of the mesh in the cage, theconductivity of the cage material, its thickness, and other variables.Stents do act as Faraday Cages in that they screen the stent lumen fromthe incident RF pulses of the MRI scanner. This prevents the protonspins of water molecules in the stent lumen from being flipped orexcited. Consequently, the desired signal from the stent lumen isreduced by this diminution in excitation. Furthermore, the stent FaradayCage likely impedes the escape of whatever signal is generated in thelumen. The stent=s high magnetic susceptibility, however, perturbs themagnetic field in the vicinity of the implant. This alters the resonancecondition of protons in the vicinity, thus leading to intravoxeldephasing with an attendant loss of signal. The net result with currentmetallic stents, most of which are stainless steel, is a signal void inthe MRI images. Other metallic stents, such as those made from Nitinol,also have considerable signal loss in the stent lumen due to acombination of Faraday Cage and magnetic susceptibility effects.

At this time, MRI is being used to non-invasively image many regions ofthe vasculature. The comprehensive cardiac MRI exam has demonstratedclinical utility in the areas of overall cardiac function, myocardialwall motion, and myocardial perfusion. It may become the standarddiagnostic tool for heart disease. With these advances in imagingtechnologies, a stent that can be meaningfully imaged by MRI in anoptimal manner would be advantageous. A non-metallic stent obviouslysolves the imaging problem. Metals, however, are the preferred materialas they make strong, low profile stents possible. Unfortunately, mostmetal stents, particularly of stainless steel, obliterate MRI images ofthe anatomy in their vicinity and obscure the stent lumen in the image.By reducing the amount of metal in the stent, or by making the cellslarger, or by having fewer cells, the Faraday Cage effect may bereduced. The RF radiation used in MRI has a wavelength of 2 to 35 metersdepending on the scanner and environment of the stent. Therefore, thecell sizes of stents are already much smaller than the RF wavelength.Increasing the stent cell size would work only primarily by decreasingthe amount of metal. This solution is limited by the need for stents tohave adequate radial strength and scaffolding.

MRI will not suddenly replace x-ray based fluoroscopy. Being new to thecardiology and interventional fields, and being an expensive technology,MRI utilization and implementation will vary by medical specialty,medical institution, and even on a country by country basis. Therefore,it seems likely that any stent produced for commercialization wouldideally be imageable by both fluoroscopy and MRI. Although theparamagnetic or ferromagnetic compounds added for MRI visibility willincrease the radiopacity of the polymer, it is not necessarily the casethat a single concentration, of a single material, will give idealvisibility in both modalities.

However, MRI has the potential to supplant, and potentially substitutefor fluoroscopy in the future. Stents which are more compatible withthis imaging modality, or which have a dual functionality, may have aclinical performance benefit. Both the future of stent materials, andthe imaging modalities used to visualize them are areas of intenseresearch due to the clinical value and large market for stents,particularly coronary stents. Although metal alloy stents currentlydominate the marketplace, polymer stents have potential advantages inthe areas of hemocompatibility, biodegradability, and drug delivery.

What is needed therefore is a stent that overcomes the shortcomingsinherent in previously known devices. Preferably, such a stent would beformed of a hybrid material, possess the required mechanicalcharacteristics, and also be readily visible using MRI, computertomography, and x-ray based fluoroscopy procedures.

SUMMARY OF THE INVENTION

The present invention is directed to a stent that overcomes theshortcomings of previously known devices by embodying a polymericmaterial combined with a metallic material to improve visibility underMRI, computer tomography and fluoroscopy.

In one embodiment of the stent of the present invention, metallic ringsare positioned between an outer layer of a first polymeric material andan inner layer of a second polymeric material. In other words, themetallic rings are sandwiched in between the first and second polymericmaterials. The rings are connected by links which are formed by thefirst and second polymeric materials. The metallic rings generally arevisible under fluoroscopy while the polymeric material provides goodvisibility using MRI.

The first and second polymeric materials can be taken from a group ofpolymeric materials which includes polyetheretherketone (PEEK), ethylvinyl alcohol (EVOH), polyetherketone, polymethylmethacrylate,polycarbonate, polyphenylenesulfide, polyphenylene, polyvinylfluoride,polyvinylidene fluoride, polypropylene, polyethylene, poly(vinylidenefluoride-co-hexafluoropropylene), poly(ethylene-co-hexafluoropropylene),poly(tetrafluoroethyelene-co-hexafluoropropylene),poly(tetrafluoroethyelene-co-ethylene), polyethyleneterephthalate,polyimides and polyetherimide. Other polymeric materials that could beused to form the inner or outer polymeric material include ePTFE,polyurethanes, polyetherurethanes, polyesterurethanes, silicone,thermoplastic elastomer (e.g. C-flex), polyether-amide thermoplasticelastomer (e.g., Pebax), fluoroelastomers, fluorosilicone elastomer,styrene-butadiene-styrene rubber, styrene-isoprene-styrene rubber,polybutadiene, polyisoprene, neoprene, ethylene-propylene elastomer,chlorosulfonated polyethylene elastomer, butyl rubber, polysulfideelastomer, polyacrylate elastomer, nitrile rubber, a family ofelastomers composed of styrene, ethylene, propylene, aliphaticpolycarbonate polyurethane, polymers augmented with antioxidants,polymers augmented with image enhancing materials, polymers having aproton (H+) core, butadiene and isoprene (e.g., Kraton) and polyesterthermoplastic elastomer (e.g., Hytrel) and a di-block co-polymer of PETand caprolactone. For strength, the polymer may further containreinforcements such as glass fiber, carbon fiber, Spectra™, or Kevlar™.

The metallic rings of each of the embodiments of the present inventionare formed from materials that are visible under fluoroscopy, such asmetallic alloys. The metallic alloys can be self-expanding or balloonexpandable and can include stainless steel, titanium, nickel-titanium,tantalum, cobalt-chromium, and the like.

In another embodiment, in order to provide higher visibility underfluoroscopy, the polymeric materials are compounded with an appropriateradiopacifier such as the powder of barium sulfate, bismuthsubcarbonate, bismuth trioxide, bismuth oxychloride, tungsten, tantalum,iridium, gold, or other dense metal. To define a biodegradablestructure, the polymeric materials are compounded with a biodegradableradiopacifier that renders it visible under fluoroscopy and can besafely released in the body. Such radiopacifiers include particles of aniodinated contrast agent and bismuth salts.

The first and second polymeric materials also can be formed ofself-expanding polymers including shape memory polymers such as oligo(e-caprolactone), dimethylacrelate, and n-butyl acrylate.

Each embodiment of the present invention also can include a therapeuticdrug or therapeutic agent associated with the first and second polymericmaterials. For example, one or more therapeutic drugs can be loaded intoeither or both of the first and second polymeric materials to prevent orreduce the likelihood of restenosis or to otherwise treat the vessel orartery.

The hybrid stent of the present invention can be made in several ways.In one embodiment, the metallic rings are cut by a laser usingconventional laser cutting procedures. The rings are then mounted on aninner polymeric tube which has been premounted on a teflon mandrel.After the rings have been mounted and positioned on the inner polymerictube, an outer polymeric tube is mounted over the metallic rings and theinner polymeric tube. A shrink tubing is then mounted over the outerpolymeric tube and it is subjected to laser bonding so that the shrinktubing contracts and applies pressure to the outer polymeric tubecausing it to compress against the metallic rings and the innerpolymeric tube. Further, heat from the laser causes the outer and innerpolymeric tubes to partially melt and fuse together. An appropriatebonding agent can be used to help adhere the inner and outer tubestogether. The shrink tubing and the supporting teflon mandrel areremoved and the stent pattern is then formed by a laser to removeunwanted portions of the polymer material, so that a pattern of metallicrings encased by the polymer material are attached to each other bypolymeric links as previously disclosed.

In another embodiment to make the hybrid stent of the present invention,a mandrel is first dip coated into a polymer which corresponds to theinner polymeric material. The metallic rings, which previously werelaser cut from a tube, are mounted on the inner polymer material andpositioned to form the stent pattern. The outer layer or outer polymermaterial is deposited over the metal rings either by spray coating or bydip coating the outer polymeric material over the rings and the innerpolymeric material. The mandrel is removed and the unwanted portions ofthe polymers can be machined by using laser cutting as previouslydescribed.

It is to be recognized that the stent of the present invention can beself-expanding or balloon-expanded. Moreover, the present invention canbe modified to be used in other body lumens including highly tortuousand distal vasculature.

These and other features and advantages of the present invention willbecome apparent from the following detailed description, which whentaken in conjunction with the accompanying drawings, illustrate by wayof example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevation view, partially in section, of theballoon-expandable hybrid stent of the invention mounted on arapid-exchange delivery catheter and positioned within an artery.

FIG. 2 is an elevation view, partially in section, similar to that shownin FIG. 1, wherein the hybrid stent is expanded within the artery sothat the stent embeds within the arterial wall.

FIG. 3 is an elevation view, partially in section, showing the expandedhybrid stent implanted within the artery after withdrawal of therapid-exchange delivery catheter.

FIG. 4A is a plan view of a flattened stent which illustrates thepattern of the hybrid stent shown in FIGS. 1-3.

FIG. 4B is a plan view of a flattened stent which illustrates the hybridstent with some straight polymeric links.

FIG. 4C is a cross-sectional view taken along lines 4C-4C depicting thehybrid strut in one of the stent rings.

FIG. 4D is a cross-sectional view taken along lines 4D-4D depicting apolymer link.

FIG. 4E is a cross-sectional view taken along lines 4E-4E depicting apolymer link having a cavity containing a therapeutic drug or agent.

FIG. 5 is a perspective view of a pre-expanded balloon expandable hybridstent depicting the cylindrical wall defined by the cylindrical rings.

FIG. 6 is an enlarged sectional view of FIG. 4 depicting a W-shapedportion of a cylindrical ring.

FIG. 7 is an enlarged sectional view of FIG. 4 depicting severalU-shaped or undulating peaks of a cylindrical ring.

FIG. 8 is an enlarged sectional view of FIG. 4 depicting a Y-shapedportion of the cylindrical ring.

FIG. 9 is a plan view of the hybrid stent in a flattened and expandedcondition.

FIG. 10 is a typical stress-strain curve for a superelastic material.

FIG. 11 is a longitudinal cross-sectional view depicting the metallicrings and polymer materials being formed on a mandrel.

FIG. 12 is a longitudinal cross-sectional view depicting the metallicrings and polymer materials being formed on a mandrel and the unwantedportions being removed by a laser.

FIG. 13 is a longitudinal cross-sectional view depicting the metallicrings and polymer materials being formed on a mandrel and the unwantedportions being removed by a laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The hybrid stent of the present invention combines the features andadvantages of metallic stents with those of polymeric stents so that thecombined hybrid stent provides the required structural support for avessel such as a coronary artery, yet is visible under any of MRI,computer tomography or x-ray fluoroscopy. In general, metallic rings arealigned along a stent longitudinal axis and an outer layer of a firstpolymeric material covers the outer surface of the metallic rings, andan inner layer of a second polymeric material covers the inner surfaceof the metallic rings. At least one link connects adjacent metallicrings whereby the links are formed by the inner and outer polymericmaterials. Virtually any stent pattern, of the many known stentpatterns, can be used to form the hybrid stent of the present invention.Thus, while certain embodiments of the hybrid stent are describedherein, they are for exemplary purposes only, and are not meant to belimiting.

Turning to the drawings, FIG. 1 depicts a hybrid stent 10 mounted on acatheter assembly 12 which is used to deliver the stent and implant itin a body lumen, such as a coronary artery, peripheral artery, or othervessel or lumen within the body. The catheter assembly includes acatheter shaft 13 which has a proximal end 14 and a distal end 16. Thecatheter assembly is configured to advance through the patient=svascular system by advancing over a guide wire by any of the well knownmethods of an over-the-wire system (not shown) or a well known rapidexchange catheter system, such as the one shown in FIG. 1.

Catheter assembly 12, as depicted in FIG. 1, includes an RX port 20where the guide wire 18 will exit the catheter. The distal end of theguide wire 18 exits the catheter distal end 16 so that the catheteradvances along the guide wire on a section of the catheter between theRX port 20 and the catheter distal end 16. As is known in the art, theguide wire lumen which receives the guide wire is sized for receivingvarious diameter guide wires to suit particular applications. The stentis mounted on the expandable member 22 (a catheter balloon) and iscrimped tightly thereon so that the stent and expandable member presenta low profile diameter for delivery through the arteries.

As shown in FIG. 1, a partial cross-section of an artery 24 contains asmall amount of plaque that has been previously treated by anangioplasty or other repair procedure. Stent 10 is used to repair adiseased or damaged arterial wall or a dissection or flap, which arecommonly found in the coronary arteries and other blood vessels.

In a typical procedure to implant stent 10, the guide wire 18 isadvanced through the patient=s vascular system by well known methods sothat the distal end of the guide wire is advanced past the plaque ordiseased area 26. Prior to implanting the stent, the cardiologist maywish to perform an angioplasty or other procedure (e.g., atherectomy) inorder to open and remodel the vessel and the diseased area. Thereafter,the stent delivery catheter assembly 12 is advanced over the guide wireso that the stent is positioned in the target area. The expandablemember or balloon 22 is inflated by well known means so that it expandsradially outwardly and in turn expands the stent radially outwardlyuntil the stent is supporting the vessel wall. The expandable member isthen deflated and the catheter withdrawn from the patient=s vascularsystem. The guide wire typically is left in the lumen forpost-dilatation procedures, if any, and subsequently is withdrawn fromthe patient=s vascular system. As depicted in FIGS. 2 and 3, the balloonis fully inflated with the stent expanded and pressed against the vesselwall, and in FIG. 3, the implanted stent remains in the vessel after theballoon has been deflated and the catheter assembly and guide wire havebeen withdrawn from the patient.

Due to the formation of the metallic portion of the hybrid stent 10 froma tube, the straight and undulating components of the stent arerelatively flat in transverse cross-section, so that when the stent isexpanded, it is pressed into the wall of the artery and as a result doesnot interfere with the blood flow through the artery. The stent ispressed into the wall of the artery and will eventually be covered withendothelial cell growth which further minimizes blood flow interference.The undulating portion of the stent provides good tackingcharacteristics to prevent stent movement within the artery.Furthermore, the cylindrical rings are closely spaced at regularintervals to provide uniform support for the wall of the artery, andconsequently are well adapted to tack up and hold in place small flapsor dissections in the wall of the artery.

Turning to FIG. 4A, one embodiment of the hybrid stent 10 is shown asflat sheet, so that the pattern can be clearly viewed, even though thestent is not in this form when in use. The stent is typically formedfrom a tubular member, although it can be formed from a flat sheet androlled into a cylindrical configuration or by other known means.

The hybrid stent 10 of the present invention, as shown in FIGS. 4-9,includes a metallic ring material 30 that provides structural supportand vessel wall coverage. The metallic ring material is sandwichedthrough an outer polymeric material 31 and inner polymeric material 32so that the metallic ring material is completely encased in the twopolymeric layers or materials. Portions of the polymeric materials areremoved to form links 33 which then connect the metallic ring material.The overall structure thus includes metallic rings connected by links,whereby the metallic rings are formed of a metal alloy sandwichedbetween an outer polymeric material and an inner polymeric material andthe links which are formed of the outer polymeric material and the innerpolymeric material. As shown, for example, in FIGS. 4C and 4D, themetallic ring material is sandwiched between an outer polymeric materialand an inner polymeric material (FIG. 4C) while the cross-sectional viewof the links (FIG. 4D) shows an outer polymeric material and an innerpolymeric material bonded together. The polymeric links will increasethe longitudinal and flexural flexibility of the stent which facilitatesdelivery of the stent through tortuous lumens, such as the coronaryarteries or vessels in the brain. Further, the polymeric links willpermit the stent to conform with arterial walls after expansion. Thenumber of metallic rings and the number of connecting links can varydepending upon the application, and since the polymeric links aretypically more flexible than prior art metallic links, more of thepolymeric links connecting each ring are available, which provides moresupport for the vessel wall. Additionally, since the polymeric links arehighly flexible, comparatively more metallic rings can be used than withconventional all metallic stents without compromising the overallflexibility of the stent. By increasing the number of polymeric linksand the number of metallic rings, the amount of scaffolding or wallcoverage provided by the stent is greatly increased, thereby reducinginstances of plaque prolapse.

In one embodiment, the inner polymeric material 32 is made frompolyetheretherketone (PEEK). PEEK offers a higher flexibility comparedto most metallic alloys, for example, PEEK has a module of elasticity(E) of 4 GPa compared to stainless steel which has an E of 200 GPa.Further, PEEK has excellent radiographic qualities which includeelimination of imaging artifacts and scatter generated from metallicimplants which would prevent a complete visualization of tissue whenusing conventional imaging techniques such as x-ray, computer tomographyor MRI. Since PEEK is MRI compatable, MRI technologies can be used toscan patients implanted with the hybrid stent 10 of the presentinvention having an inner polymeric material made from PEEK. Further,PEEK can be modified to enhance its radiopacity. For example,PEEK-OptimaJ (made by Invibio Biomaterial Solutions, Greenville, S.C.)has been modified to be radiopaque under fluoroscopy and is suitable forimplant applications. The outer polymeric material 31 can be a polymersuch as ethyl benyl alcohol (EVOH; available commercially as EVAL7, EVALCompany of America, Lisle, Ill.), which again provides flexibility thatis comparable to PEEK, and is substantially more flexible than stainlesssteel, for example. The first and second polymeric materials can betaken from a group of polymeric materials which includespolyetheretherketone (PEEK), ethyl vinyl alcohol (EVOH),polyetherketone, polymethylmethacrylate, polycarbonate,polyphenylenesulfide, polyphenylene, polyvinylfluoride, polyvinylidenefluoride, polypropylene, polyethylene, poly(vinylidenefluoride-co-hexafluoropropylene), poly(ethylene-co-hexafluoropropylene),poly(tetrafluoroethyelene-co-hexafluoropropylene),poly(tetrafluoroethyelene-co-ethylene), polyethyleneterephthalate,polyimides and polyetherimide. Other polymeric materials that could beused to form the inner or outer polymeric material include ePTFE,polyurethanes, polyetherurethanes, polyesterurethanes, silicone,thermoplastic elastomer (e.g. C-flex), polyether-amide thermoplasticelastomer (e.g., Pebax), fluoroelastomers, fluorosilicone elastomer,styrene-butadiene-styrene rubber, styrene-isoprene-styrene rubber,polybutadiene, polyisoprene, neoprene, ethylene-propylene elastomer,chlorosulfonated polyethylene elastomer, butyl rubber, polysulfideelastomer, polyacrylate elastomer, nitrile rubber, a family ofelastomers composed of styrene, ethylene, propylene, aliphaticpolycarbonate polyurethane, polymers augmented with antioxidants,polymers augmented with image enhancing materials, polymers having aproton (H+) core, butadiene and isoprene (e.g., Kraton) and polyesterthermoplastic elastomer (e.g., Hytrel) and a di-block co-polymer of PETand caprolactone. For strength, the polymer may further containreinforcements such as glass fiber, carbon fiber, Spectra™, or Kevlar™.

In one embodiment, the outer polymeric material 31 and/or the innerpolymeric material 32 are loaded with a therapeutic agent or drug fortreating the artery or vessel in which the hybrid stent 10 is implanted.As an example, an anti-restenotic drug can be loaded into the PEEK orEVOH of the outer polymeric material or the inner polymeric material toreduce the likelihood of the development of restenosis of a coronaryartery. In another embodiment, as shown in FIG. 4E, a cavity 34 isformed between the outer polymeric material 31 and the inner polymericmaterial 32 for containing a therapeutic drug 35. The drug would elutethrough the polymeric material at a controlled rate to treat the arteryor vessel in which the stent is implanted. In the embodiment shown inFIG. 4E, cavity 34 is positioned between the first polymeric materialand the second polymeric material in the polymeric link 33. One or morelinks can be formed to create the cavity and then loaded with atherapeutic agent or drug in order to prevent the development ofrestenosis, or to treat the artery or vessel for other conditions, suchas to reduce clot formation.

Examples of therapeutic agents or drugs that are suitable for use withthe outer and inner polymeric materials 31,32 include rapamycin,actinomycin D (ActD), or derivatives and analogs thereof. ActD ismanufactured by Sigma-Aldrich, 1001 West Saint Paul Avenue, MilwaukeeWis. 53233, or COSMEGEN, available from Merck. Synonyms of actinopmycinD include dactinomycin, actinomycin IV, actinomycin 11, actinomycin X1,and actinomycin C1. Examples of agents include other antiproliferativesubstances as well as antineoplastic, antinflammatory, antiplatelet,anticoagulant, antifibrin, antithomobin, antimitotic, antibiotic, andantioxidant substances. Examples of antineoplastics include taxol(paclitaxel and docetaxel). Examples of antiplatelets, anticoagulants,antifibrins, and antithrombins include sodium heparin, low molecularweight heparin, hirudin, argatroban, forskolin, vapiprost, prostacyclinand prostacyclin analogs, dextran, D-phe-pro-arg-chloromethylketone(synthetic antithrombin), dipyridamole, glycoprotein, 11b/111a plateletmembrane receptor antagonist, recombinant hirudin, thrombin inhibitor(available from Biogen), and 7E-3B7 (an antiplatelet drug fromCentocore). Examples of antimitotic agents include methotrexate,azathioprine, vincristine, vinblastine, fluorouracil, adriamycin, andmutamycin. Examples of cytostatic or antiproliferative agents includeangiopeptin (a somatostatin analog from Ibsen), angiotensin convertingenzyme inhibitors such as Captopril (available from Squibb), Cilazapril(available from Hoffman-LaRoche), or Lisinopril (available from Merck);calcium channel blockers (such as Nifedipine), colchicine fibroblastgrowth factor (FGF) antagonists, fish oil (omega 3-fatty acid),histamine antagonist, Lovastatin (an inhibitor of HMG-CoA reductase, acholesterol lowering drug from Merck), monoclonal antibodies (such asPDGF receptors), nitroprusside, phosphodiesterase inhibitors,prostaglandin inhibitor (available from Glazo), Seramin (a PDGFantagonist), serotonin blockers, steroids, thioprotease inhibitors,triazolopyrimidine (a PDGF antagonist), and nitric oxide. Othertherapeutic substances or agents which may be appropriate includealpha-interferon, genetically engineered epithelial cells, anddexamethasone.

The metallic ring material 30 of the present invention can includealloys such as stainless steel, titanium, tantalum, nickel-titanium,cobalt-chromium, tungsten, and similar metallic alloys that arebiocompatible and that are generally known for use as intravascularstents.

The hybrid stent 10 of the present invention can be incorporated in awide range of stent patterns that are widely known and currentlyavailable. Many of the currently available stent patterns include somecombination of cylindrical rings that have an undulating shape that iscrimpable onto the balloon portion of a catheter and expandable forimplanting in an artery or a vessel. Typically, the rings are connectedby connecting members or links with both the rings and links beingformed of the same metal, such as stainless steel, titanium, orcobalt-chromium. Thus, the hybrid stent of the present invention can beused to make any of the following commercially available stents that goby the trade names ACS Multi-Link7 family of stents made by AdvancedCardiovascular Systems, Inc., Santa Clara, Calif.; the BX VelocityStent7 made by Cordis, a Johnson & Johnson company, Warren, N.J.; theAVE S6707 and S77 stents made by AVE, a division of MedtronicCorporation, Minneapolis, Minn.; and the NIR7 and the Express7 stentsmade by Medinol and distributed in the United States by BostonScientific, Corp., Natick, Mass. As a further example, an ACSMulti-Link7 PENTA stent is further described herein for purposes ofillustration to demonstrate its conformability to the hybrid stent 10 ofthe present invention.

In keeping with the invention, as shown in FIGS. 4-9, one embodiment ofthe hybrid stent 10 is made up of cylindrical rings 40 which extendcircumferentially around the stent when it is in a tubular form. Thestent has a delivery diameter 42 as shown in FIG. 5, and an implanteddiameter 44 as shown in FIG. 3. Each cylindrical ring 40 has acylindrical ring proximal end 46 and a cylindrical ring distal end 48.Typically, since the stent is laser cut from a tube, there are nodiscreet parts such as the described cylindrical rings. However, it isbeneficial for identification of various parts to refer to thecylindrical rings and other parts of the stent.

Each cylindrical ring 40 defines a cylindrical plane 50 which is a planedefined by the proximal and distal ends 46, 48 and the circumferentialextent as the cylindrical ring travels around the cylinder. Eachcylindrical ring includes cylindrical outer wall surface 52 whichdefines the outermost surface of the stent, and cylindrical inner wallsurface 53 which defines the innermost surface of the stent. Cylindricalplane 50 follows the cylindrical outer wall surface.

Undulating link 54 is positioned within cylindrical plane 50 such thatthe undulating links connect one cylindrical ring to an adjacentcylindrical ring and provide overall longitudinal flexibility to thestent due to their unique construction. The flexibility of undulatinglinks derives in part from bends 56 connected to straight portions 58wherein the straight portions are substantially perpendicular to thelongitudinal axis of the stent. Thus, as the stent is being deliveredthrough a tortuous vessel, such as a coronary artery, the bends 56 andstraight portions 58 of the undulating links will permit the stent toflex in the longitudinal direction which substantially enhances deliveryof the stent to the target site. The number of bends and straightportions can be increased or decreased from that shown, to achievediffering flexibility constructions. With the straight portions beingsubstantially perpendicular to the stent longitudinal axis, theundulating link acts like a hinge to provide flexibility.

Referring to FIGS. 4A, 4B and 5, the stent 10 can be described moreparticularly as having a plurality of peaks 70 and valleys 72. Althoughthe stent is not divided into separate elements, for ease of discussionreferences to peaks and valleys is appropriate. The number of peaks andvalleys, sometimes referred to as crowns, can vary in number for eachring, depending upon the application. Thus, for example, if the stent isto be implanted in a coronary artery, a lesser number of peaks andvalleys (or crowns) are required than if the stent is implanted in aperipheral artery, which has a larger diameter than a coronary artery.As can be seen in FIGS. 4A and 4B, peaks 70 are substantially in phasewhen looking at every other cylindrical ring 40. The in-phaserelationship is identified by reference number 74. It may be desirableunder certain circumstances to position peaks 70 so that they are inphase in every cylindrical ring (not shown). As shown in FIG. 4A, thepeaks are circumferentially offset from the valleys and from theundulating link 54. Positioning the peaks, valleys, and undulating linksin this manner provides a stent having uniform expansion capabilities,high radial strength, a high degree of flexibility, and sufficient wallcoverage to support the vessel.

Referring to FIGS. 6-9, the stent can be described as having U-shapedportions 90, Y-shaped portions 92, and W-shaped portions 94. Again,while the cylindrical rings are generally laser cut from a tube and thestent typically has no discreet parts, for ease of identification thestent of the invention also can be referred to as having U-, Y-, andW-shaped portions. The U-shaped portions have no supporting structureattached thereto. The Y-shaped portions, at their base, or apex, havearm 95 extending therefrom and attached to undulating link 54. The Wportion has at its base or curve portion arm 96 which attaches at theother end of the undulating link. The length of the arms attaching thelinks to the rings can vary. Importantly, the arms should be sized inconjunction with the undulating link. Preferably, undulating link 54 iscontained within W-shaped portion 94, which should be wide enough toaccommodate the undulating link when the stent is crimped so that noportion of the undulating link and the W-portion overlap. Preferably,the undulating link and the W-shaped portion are in the same cylindricalplane 50 as defined by the cylindrical outer wall surface 52 and thecylindrical inner wall surface 53. FIG. 9 shows one embodiment of thehybrid stent 10 in a flattened configuration and expanded. Again, thestent normally is not in this configuration (flattened and expanded) andshown for illustration purposes to better understand the invention.

One embodiment of the hybrid stent 10 of the present invention includesa superelastic material in the metallic rings and/or the polymericlinks. The term “superelastic” refers to an isothermal transformation,more specifically stress inducing a martensitic phase from an austeniticphase. Alloys having superelastic properties generally have at least twophases: a martensitic phase, which has a relatively low tensile strengthand which is stable at relatively low temperatures, and an austeniticphase, which has a relatively high tensile strength and which is stableat temperatures higher than the martensitic phase. The austenitic phasealso typically has better corrosion properties. Superelasticcharacteristics generally allow the metal stent to be deformed bycollapsing and deforming the stent and creating stress which causes themetal to change to the martensitic phase. The stent is restrained in thedeformed condition to facilitate the insertion into a patient's body,with such deformation causing the phase transformation. Once within thebody lumen, the restraint on the stent is removed, thereby reducing thestress therein so that the superelastic stent can return towardsoriginal undeformed shape by the transformation back to the austeniticphase. A basic discussion of this phenomenon can be found in Wayman andDeuring, An Introduction to Martensite and Shape Memory, @ which appearsin Engineering Aspects Of Shape Memory Alloys, Deuring et al. editors(Butterworth-Heinemann Ltd. 1990, Great Britain.)

In this embodiment, the cylindrical ring material 30 of the hybrid stent10 are formed from a superelastic material such as NiTi and undergo anisothermal transformation when stressed. The stent is first compressedto a delivery diameter, thereby creating stress in the NiTi alloy sothat the NiTi is in a martensitic state having relatively low tensilestrength. While still in the martensitic phase, the stent is mountedonto a catheter by known methods.

When stress is applied to a specimen of a metal such as nitinolexhibiting superelastic characteristics at a temperature at or abovethat which the transformation of the martensitic phase to the austeniticphase is complete, the specimen deforms elastically until it reaches aparticular stress level where the alloy then undergoes a stress-inducedphase transformation from the austenitic phase to the martensitic phase.As the phase transformation progresses, the alloy undergoes significantincreases in strain with little or no corresponding increases in stress.The strain increases while the stress remains essentially constant untilthe transformation of the austenitic phase to the martensitic phase iscomplete. Thereafter, further increase in stress is necessary to causefurther deformation. The martensitic metal first yields elastically uponthe application of additional stress and then plastically with permanentresidual deformation.

If the load on the specimen is removed before any permanent deformationhas occurred, the martensite specimen will elastically recover andtransform back to the austenitic phase. The reduction in stress firstcauses a decrease in strain. As stress reduction reaches the level atwhich the martensitic phase transforms back into the austenitic phase,the stress level in the specimen will remain essentially constant (butless than the constant stress level at which the austenitic crystallinestructure transforms to the martensitic crystalline structure until thetransformation back to the austenitic phase is complete); i.e., there issignificant recovery in strain with only negligible corresponding stressreduction. After the transformation back to austenite is complete,further stress reduction results in elastic strain reduction. Thisability to incur significant strain at relatively constant stress uponthe application of a load and to recover from the deformation upon theremoval of the load is commonly referred to as superelasticity.

The prior art makes reference to the use of metal alloys havingsuperelastic characteristics in medical devices which are intended to beinserted or otherwise used within a patient=s body. See, for example,U.S. Pat. No. 4,665,905 (Jervis) and U.S. Pat. No. 4,925,445 (Sakamotoet al.), which are incorporated by reference herein in their entirety.

FIG. 10 illustrates an example of a stress-strain relationship of analloy specimen, the cylindrical rings having superelastic properties aswould be exhibited upon tensile testing of the specimen. Therelationship is plotted on x-y axis, with the x axis representing strainand the y axis representing stress. For ease of illustration, the x-yaxes are labeled with typical pseudoelastic nitinol stress from 0 to 60ksi and strain from 0 to 9 percent, respectively.

Looking at the plot itself in FIG. 10, the line from point A to point Brepresents the elastic deformation of the specimen. After point B thestrain or deformation is no longer proportional to the applied stressand it is in the region between point B and point C that thestress-induced transformation of the austenitic phase to the martensiticphase begins to occur. There also can be an intermediate phase, calledthe rhombohedral phase, depending upon the composition of the alloy. Atpoint C moving toward point D, the material enters a region ofrelatively constant stress with significant deformation or strain. Thisconstant or plateau region is known as the loading stress, since itrepresents the behavior of the material as it encounters continuousincreasing strain. It is in this plateau region CD that thetransformation from austenite to martensite occurs.

At point D the transformation to the martensitic phase due to theapplication of stress to the specimen is substantially complete. Beyondpoint D the martensitic phase begins to deform, elastically at first,but, beyond point E, the deformation is plastic or permanent.

When the stress applied to the superelastic metal is removed, thematerial behavior follows the curve from point E to point F. Within theE to F region, the martensite recovers its original shape, provided thatthere was no permanent deformation to the martensitic structure. Atpoint F in the recovery process, the metal begins to transform from thestress-induced, unstable, martensitic phase back to the more stableaustenitic phase.

In the region from point G to point H, which is also an essentiallyconstant or plateau stress region, the phase transformation frommartensite back to austenite takes place. This constant or plateauregion GH is known as the unloading stress. The line from point I to thestarting point A represents the elastic recovery of the metal to itsoriginal shape.

Binary nickel-titanium alloys that exhibit superelasticity have anunusual stress-strain relationship as just described and as plotted inthe curve of FIG. 10. As emphasized above, the superelastic curve ischaracterized by regions of nearly constant stress upon loading,identified above as loading plateau stress CD and unloading plateaustress GH. Naturally, the loading plateau stress CD is always largerthan the unloading plateau stress GH. The loading plateau stressrepresents the period during which martensite is being stress-induced infavor of the original austenitic crystalline structure. As the load isremoved, the stress-induced martensite transforms back into austenitealong the unloading plateau stress part of the curve. The difference instress between the stress at loading CD and unloading stress GH definesthe hysteresis of the system. This is identified as Δy of the curve inFIG. 10.

The present invention seeks to minimize the hysteresis of thesuperelastic material used for the self-expanding rings of the hybridstent. The rings are designed to perform various mechanical functionswithin a lumen, all of which are based upon the lower unloading plateaustress GH. Unloading plateau stress GH represents the behavior of thenitinol material when the stent is deployed.

On the other hand, the higher loading plateau stress CD establishes themechanical resistance the cylindrical rings exert against the deliverysystem, and specifically delivery sheath or restraint. It represents thestress exerted by the end-rings when they are loaded into a restraint.The greater the difference between the two plateaus CD and GH is (thehysteresis), the stronger the delivery system must be to accommodate anygiven level of stent performance.

Conversely, reducing the difference or Δy between the two plateaus CDand GH results in smaller hysteresis. The smaller the hysteresis is, thesmaller and lower profile the delivery system has to be to accommodateany given level of stent performance. Furthermore, the present inventiondelivery system can be smaller and constructed to a smaller profile dueto the lower loading plateau stress CD, while maintaining a high hoopstrength of the deployed, expanded stent represented by plateau stressGH.

The superelastic alloy is preferably formed from a compositionconsisting essentially of about 30 to about 52 percent titanium and thebalance nickel and up to 10 percent of one or more additional ternaryalloying elements. Such ternary alloying elements may be selected fromthe group consisting of palladium, platinum chromium, iron, cobalt,vanadium, manganese, boron, copper, aluminum, tungsten, or zirconium. Inparticular, the ternary element may optionally be up to 3 percent eachof iron, cobalt, platinum, palladium, and chromium, and up to about 10percent copper and vanadium. As used herein, all references to percentcomposition are atomic percent unless otherwise noted. Platinum is thepreferred ternary element at 7.5 atomic percent.

In another embodiment, a NiTi cylindrical ring with SME (shape memoryeffect) is heat-treated at approximately 500 E C. The stent ismechanically deformed into a first, smaller diameter for mounting on acatheter delivery system that includes an expandable balloon andinflation lumen. After the stent has been expanded by the balloon anddeployed against arterial wall, 45 E C heat is applied causing thecylindrical rings to return to their fully expanded larger diameter andto be in contact with the arterial wall of the artery. The applicationof 45 E C of heat is compatible with most applications in the humanbody, but it is not to be limited to this temperature, as higher orlower temperatures are contemplated without departing from theinvention. The 45 E C temperature can be achieved in a conventionalmanner well known in the art, such as by warm saline injected into thedelivery catheter and balloon.

In the embodiment in which the cylindrical ring material 30 is formed ofa superelastic or shape memory effect metal alloy, such as NiTi, one orboth of the polymers also can be formed of a shape memory polymer.Various shape memory polymers for biomedical applications can includeoligo (e-caprolactone), dimethacrylate and n-butyl acrylate. Both ofthese shape memory polymers are monomeric compounds which, whencombined, generate a family of polymers that exhibit excellent memorycharacteristics. The oligo (e-caprolactone) dimethacrylate furnishes thecrystalisable Aswitching@ segment (which is characteristic of shapememory materials changing from one Aphase@ to another) that determinesboth the temporary and the permanent shape of the polymer. By analogy,the temporary shape is similar to the martensitic phase of a metallicNiTi material, while the permanent shape of the polymer is similar tothe austentic phase of the NiTi alloy. By varying the amount of thecomonomer, n-butyl acrylate, in the polymer compound, the cross-linkdensity can be varied. This allows the mechanical strength and thetransition temperature of the polymers to be tailored over a wide range,which can be coordinated with the NiTi alloy to provide the requiredshape memory characteristics of the overall stent having both metallicrings and polymer links.

The hybrid stent of the present invention can be manufactured in manyways. In one embodiment, as shown in FIGS. 11-13, the metallic ringmaterial 30 is sandwiched between or encased within outer polymericmaterial 31 and inner polymeric material 32. In keeping with the methodof making the hybrid stent, in one embodiment, the metallic ringmaterial 30 is cut from a tube in a conventional manner by a lasercutting procedure. One such method of laser cutting metallic rings andstent patterns in general is described in U.S. Pat. No. 5,759,192 toSaunders, which is incorporated by reference herein. The inner polymericmaterial 32 is mounted on a mandrel 100 so that it is a tight fit, andthe metallic ring material is thereafter positioned over the innerpolymeric material. The inner polymeric material can be in the form of atube. After the metallic ring material is positioned over the innerpolymeric material, the outer polymeric material, in the form of apolymeric tube, is positioned over the metallic rings and the innerpolymeric material. A shrink tubing 102 is then mounted over the outerpolymeric material and then exposed to heat, by a laser 104 to shrinkthe shrink tubing which in turn applies compressive pressure on theouter polymeric tube. As the outer polymeric tube compresses against themetallic rings and the inner polymeric tube, the heat from the lasercauses the inner and outer polymeric materials to melt and fusetogether. To facilitate adherence between the inner and outer polymericmaterials, an appropriate adhesive or bonding agent 106 can be used inaddition to the heat process from the laser. After fusing the inner andouter polymeric materials, the shrink tubing and the mandrel are removedand a laser is used to remove unwanted portions of the polymer toproduce the desired stent pattern. For example, as previously describedwith respect to the stent pattern of FIGS. 4-9, the laser removesportions of the inner and outer polymeric material to form connectinglinks between the metallic ring material. The resulting stent pattern ofrings and links can then be used in conjunction with a stent deliverycatheter as shown in FIGS. 1-3 to deliver and implant the stent in avessel or artery.

In another embodiment, as shown in FIG. 13, in which the metallic ringmaterial 30 is encased or sandwiched between the inner polymericmaterial 32 and the outer polymeric material 31, a mandrel 100 is firstdip coated with the polymer corresponding to the inner polymericmaterial. The metallic ring material is then positioned on the innerpolymeric material which is coating the mandrel. The outer layer ofpolymeric material is deposited over the metallic ring material and theinner polymeric material either by dip coating or spray coating.Thereafter, the stent having the metallic ring material encased withinthe inner and outer polymeric materials is removed from the mandrel andby use of a laser or equivalent procedure, the unwanted portions of thepolymers are removed to develop the stent pattern.

While the invention has been illustrated and described herein in termsof its use as an intravascular stent, it will be apparent to thoseskilled in the art that the stent can be used in other body lumens.Further, particular sizes and dimensions, materials used, and the likehave been described herein and are provided as examples only. Othermodifications and improvements may be made without departing from thescope of the invention.

1. A stent, comprising: a plurality of metallic rings aligned along astent longitudinal axis; an outer layer of a first polymeric materialcovering an outer surface of the metallic rings; an inner layer of asecond polymeric material covering an inner surface of the metallicrings, the inner layer being bonded directly to the outer layer in areaswhere there is no metallic rings; and at least two discrete linksconnecting adjacent metallic rings together, each of the discrete linksbeing formed in the area where the outer layer is directly bonded to theinner layer.
 2. The stent of claim 1, wherein the first polymericmaterial is EVOH.
 3. The stent of claim 1, wherein the second polymericmaterial is PEEK.
 4. The stent of claim 3, wherein the PEEK isradiopaque.
 5. The stent of claim 2, wherein the EVOH is radiopaque. 6.The stent of claim 1, wherein the first and second polymeric materialsare taken from the group of polymers consisting of polyetheretherketone(PEEK), ethyl vinyl alcohol (EVOH), polyetherketone,polymethylmethacrylate, polycarbonate, polyphenylenesulfide,polyphenylene, polyvinylfluoride, polyvinylidene fluoride,polypropylene, polyethylene, poly(vinylidenefluoride-co-hexafluoropropylene), poly(ethylene-co-hexafluoropropylene),poly(tetrafluoroethyelene-co-hexafluoropropylene),poly(tetrafluoroethyelene-co-ethylene), polyethyleneterephthalate,polyimides, polyetherimide, ePTFE, polyurethanes, polyetherurethanes,polyesterurethanes, silicone, thermoplastic elastomer, polyether-amidethermoplastic elastomer, fluoroelastomers, fluorosilicone elastomer,styrene-butadiene-styrene rubber, styrene-isoprene-styrene rubber,polybutadiene, polyisoprene, neoprene, ethylene-propylene elastomer,chlorosulfonated polyethylene elastomer, butyl rubber, polysulfideelastomer, polyacrylate elastomer, nitrile rubber, a family ofelastomers composed of styrene, ethylene, propylene, aliphaticpolycarbonate polyurethane, polymers augmented with antioxidants,polymers augmented with image enhancing materials, polymers having aproton (H+) core, butadiene and isoprene and polyester thermoplasticelastomer and a di-block co-polymer of PET and caprolactone.
 7. Thestent of claim 1, wherein the metallic rings are formed of a metal alloytaken from the group of metal alloys consisting of stainless steel,titanium, tantalum, nickel-titanium, cobalt-chromium, and tungsten. 8.The stent of claim 1, wherein one or both of the first and secondpolymeric materials are loaded with a therapeutic drug.
 9. The stent ofclaim 8, wherein the therapeutic drug is taken from the group of drugsincluding one or more of everolimus, rapamycin, actinomycin D (ActD), orderivatives and analogs thereof; synonyms of actinopmycin D includingdactinomycin, actinomycin IV, actinomycin 11, actinomycin X1, andactinomycin C1; antiproliferative substances including antineoplastic,antinflammatory, antiplatelet, anticoagulant, antifibrin, antithomobin,antimitotic, antibiotic, antioxidant substances, taxol (paclitaxel anddocetaxel), anticoagulants, antifibrins, sodium heparin, low molecularweight heparin, hirudin, argatroban, forskolin, vapiprost, prostacyclinand prostacyclin analogs, dextran, D-phe-pro-arg-chloromethylketone(synthetic antithrombin), dipyridamole, glycoprotein, IIb/IIIa plateletmembrane receptor antagonist, recombinant hirudin, thrombin inhibitormethotrexate, azathioprine, vincristine, vinblastine, fluorouracil,adriamycin, mutamycin; angiopeptin, angiotensin converting enzymeinhibitors; calcium channel blockers; colchicine fibroblast growthfactor (FGF) antagonists; fish oil (omega 3-fatty acid); histamineantagonist; monoclonal antibodies (such as PDGF receptors);nitroprusside; phosphodiesterase inhibitors; prostaglandin inhibitor (aPDGF antagonist); serotonin blockers; steroids; thioprotease inhibitors;triazolopyrimidine (a PDGF antagonist); and nitric oxide.
 10. The stentof claim 1, wherein a radiopaque marker is positioned between the firstpolymeric material and the second polymeric material.
 11. The stent ofclaim 1, wherein the outer layer of the first polymeric material has athickness in the range of 0.001 mm to 2.5 mm.
 12. The stent of claim 11,wherein the outer layer has a uniform thickness.
 13. The stent of claim11, wherein the outer layer has a variable thickness.
 14. The stent ofclaim 1, wherein the inner layer of the second polymeric material has athickness in the range of 0.001 mm to 2.5 mm.
 15. The stent of claim 14,wherein the inner layer has a uniform thickness.
 16. The stent of claim14, wherein the inner layer has a variable thickness.
 17. The stent ofclaim 1, wherein the metallic rings have an undulating pattern.
 18. Thestent of claim 17, wherein the undulating pattern includes U-shapedelements.
 19. The stent of claim 1, wherein the stent is configured sothat it is visible under any of x-ray fluoroscopy, computer tomography,or MRI.
 20. The stent of claim 1, wherein a cavity is formed between thefirst polymeric material and the second polymeric material so that atherapeutic drug can be releasably contained within the cavity.
 21. Thestent of claim 1, wherein the metallic rings are formed of struts, thestruts having a substantially uniform radial thickness.
 22. The stent ofclaim 1, wherein the metallic rings are formed of struts, the strutshaving a variable radial thickness.
 23. The stent of claim 1, whereinthe first polymeric material is a shape memory polymer.
 24. The stent ofclaim 23, wherein the shape memory polymer includes the family ofpolymers oligo (e-caprolactone) dimethacrylate combined with n-butylacrylate.
 25. A stent, comprising: a plurality of metallic rings alignedalong a stent longitudinal axis, each ring having an inner surface andan outer surface; a polymeric layer covering either or both of the innerand outer surfaces of each of the metallic rings, the polymeric layerextending in an area between adjacent metallic rings; and at least twodiscrete links connecting adjacent metallic rings together, each of thediscrete links being formed by the polymeric material in the areabetween adjacent metallic rings.
 26. The stent of claim 25, wherein thepolymeric layer is made from EVOH.
 27. The stent of claim 25, whereinthe polymeric layer is made from PEEK.
 28. The stent of claim 25,wherein the discrete links connecting adjacent rings are spaced apartfrom each other along the circumference of the rings.
 29. The stent ofclaim 25, wherein the discrete links are integral with the polymericlayer covering either or both of the inner and outer surfaces of each ofthe metallic rings.
 30. The stent of claim 25, wherein the polymericlayer is loaded with a therapeutic drug.
 31. The stent of claim 25,wherein a cavity is formed on the polymeric layer so that a therapeuticdrug can be releasably contained within the cavity.
 32. The stent ofclaim 25, wherein each of the rings is made with a strut having aparticular width and the links take the form of a strut having the sameor substantially the same width as the struts forming the rings.
 33. Thestent of claim 25, wherein some of the discrete links have an undulatingpattern.
 34. A stent, comprising: a plurality of metallic rings alignedalong a stent longitudinal axis, each ring having an inner surface andan outer surface; an inner layer of polymeric material extending overthe inner surface of each of the metallic rings; an outer layer ofpolymeric material extending over the outer surface of each of themetallic rings, the inner layer being bonded directly to the outer layerin areas where there is no metallic rings; and at least two discretelinks connecting adjacent metallic rings together, each of the discretelinks being formed in the area where the inner layer is directly bondedto the outer layer of polymeric material.
 35. The stent of claim 34,wherein some of the discrete links have an undulating pattern.
 36. Thestent of claim 34, wherein each of the discrete links are integral withthe inner and outer layers of polymeric material.
 37. The stent of claim34, wherein each of the rings is made with a strut having a particularwidth and the discrete links take the form of a strut having the same orsubstantially the same width as the struts forming the rings.
 38. Thestent of claim 34, wherein the inner and outer layers of polymericmaterial forming the discrete links are bonded together.
 39. The stentof claim 34, wherein one or both of the inner and outer are loaded witha therapeutic drug.
 40. The stent of claim 34, wherein a cavity isformed between the inner layer and outer layer so that a therapeuticdrug can be releasably contained within the cavity.
 41. The stent ofclaim 40, wherein the cavity is located on at least one of the discretelinks.
 42. The stent of claim 34, wherein each discrete link connectingadjacent rings is spaced apart from each other along the circumferenceof the rings.
 43. The stent of claim 42, wherein the discrete links areintegral with the layers of polymeric material covering the inner andouter surfaces of each of the metallic rings.